The present invention relates to a miniature fuse of surface mount type. In particular, the present invention is suitable for a micro-miniature fuse of surface mount type which can be used for protection of apparatuses for communication and whose longitudinal length does not exceed 11 mm.
Apparatuses for communication to be connected to telephone lines and the like are liable to be subjected to high surge current due to indirect lightning strikes, or sudden and unusually high increases in voltage due to telephone lines accidentally coming into contact with power lines. Thus, fuses to be used for apparatuses for communication require both a strong time lag characteristic preventing the fuses from being melted by the surge current due to indirect lightning strikes as well as a high breaking capacity in the order of 60A at AC600V which assures a big fault current flow at the moment of the accidental power line contact to be safely switched off. Furthermore, as the apparatuses for communication become increasingly miniaturized, micro-miniature sized fuses are required to have a strong time lag characteristic and a high breaking capacity, they are also required to be of a surface mount type so that surface mounting of high density may be carried out. Thus far there have been provided inner soldered fuses which are constructed in such a manner as shown in FIG. 1 and FIG. 2, wherein a fusible member 102 is wound around a support member of insulating material made up of bundled glass fibers and the respective ends of the fusible member 102 are soldered to the recessed bottom of the conductive terminals 104 of cap-like configuration.
A disadvantage of the inner soldered type fuse is that the electrical resistances of produced fuses disperse widely from designed value so that the pre-arcing time may often be uneven. In accordance with an inner soldering type, when a soldering iron is placed from the outside of the cap-like terminal 104 so as to melt the solder 106 attached to the interior of the recess of the cap terminal 104 to solder the fusible member 102 wound around the bundled glass fibers 100 to the bottom of the recess of the cap terminal 104, melted soldering material will be caused to flow along the fusible member 102 wound around the bundle of the glass fibers 100, resulting in a clogging of the spaces between the adjacent portions of the wound fusible member 102 to make short-circuits between them. As a result, the length of the fusible member 102 which is in the short-circuit state may result in one-third of overall length of the fusible member, and thus the performance of fuses may be changed entirely. In addition to the above, it also often happens that at the time of breaking, the soldering material inside the cap terminal will be vaporized and arc will be sustained, making it impossible for the breaking to be performed, which is also a disadvantage.
Further, the conductive cap-like terminals 104 and the main body 108 made of an insulating material are secured by the frictional force caused by coagulation of the soldering material which has flowed into the space between the cap terminals and the main body. In the case of fuses of surface mount type, when such fuses are mounted on a substrate by means of soldering, the fuses are also heated to a temperature of soldering. Since the soldering temperature profiles differ from manufacturer to manufacturer, in the case of high temperature soldering, the soldering materials within the fuses, namely the soldering materials which have entered between the conductive cap-like terminals 104 and the main body 108 are caused to melt, whereby the conductive cap-like terminals 104 might possibly be detached from the main body 108, which was considered a problem Furthermore, since the melting point of the soldering material which does not contain lead in view of problems associated with its use tends to be relatively high, the soldering temperature at the time of mounting fuses on a substrate is likely to be further increased, which is another problem that will have to be solved.
As shown in FIG. 1 and FIG. 2, the main body 108 has a columnar configuration and a through hole 110 is so provided as to extend between the opposite end faces of the main body in the longitudinal direction. Since a miniature fuse of surface mount type is so small, in the order of 11 mm in respect of the overall length, the diameter of such a through hole is also very small. Accordingly, since the support member 100 with the fusible member 102 wound therearound had to be inserted through a small inlet on the end face of the main body 108, workability in the course of manufacturing was consequently poor.
An object of the present invention is to provide a miniature fuse of surface mount type which has a stable pre-arcing time-current characteristic without uneven performance and a strong time lag characteristic and, in addition, a large breaking capacity.
Another object of the present invention is to provide a miniature fuse of surface mount type which has a stable pre-arcing time current characteristic without uneven performance and a large breaking capacity.
A further object of the present invention is to provide a miniature fuse of surface mount type which is easy to manufacture.
The object of the present invention mentioned above may be accomplished by a miniature fuse of surface mount type according to the present invention including a fusible member, a support member adapted to support said fusible member, a main body made of heat resistant insulating material and a pair of conductive terminals, wherein said main body includes a pair of opposed end portions and a cavity defined inside of the main body between said pair of end terminals, the middle part of said fusible member is wound around said support member and, in this condition, disposed in said cavity between a pair of end portions, the opposite end portions of said fusible member are extended outwardly onto the peripheral surface of said main body from a pair of end portions of said main body or from the vicinities thereof, the respective conductive terminals are fit onto the respective end portions of said main body, and connected electrically and mechanically to the respective end portions of said fusible member, and wherein said support member is made of a material that has ease of shape formation, has pressure resistant strength, containing in terms of weight ratio Al2O3 exceeding 96%, MgO exceeding 3%, and BeO less than 1%; said fusible member is made of a metallic material having a low melting temperature and containing in terms of weight ratio Ag equal to or exceeding 50%, Cu equal to or exceeding 20%, Zn equal to or exceeding 17% and Sn equal to or exceeding 5%; and said electrical and mechanical connection is executed by welding.
According to an aspect of the present invention, it is preferable that cut-out recessed portions are formed at two locations along the diagonal line on the outer peripheral surface at the opposite end portions of said main body in contact with the end surfaces of said main body, and the respective end portions of said fusible member are engaged with said cut-out recessed portions.
According to another aspect of the present invention, it is preferable that said main body has a columnar configuration; said conductive terminals are of cap-like configuration having recessed portions to be fit onto the opposite end portions of said main body; and lids of thin sheet made of insulating material and having a thickness smaller than the depth of said recessed portions are provided between the end surfaces of said main body and the bottom of the recessed portions of said conductive terminals.
The second object of the present invention mentioned above may be accomplished by a miniature fuse of surface mount type according to the present invention including a fusible member, a main body made of heat resistant insulating material, and a pair of conductive terminals, wherein said main body includes a pair of opposing end portions and a cavity defined inside the main body between said pair of end portions, said fusible member is disposed in said cavity of said main body between said pair of end portions, the opposite end portions of said fusible member are extended outwardly onto the outer surface of said main body from a pair of end portions of said main body or from the vicinities thereof, the respective conductive terminals are fit onto the respective end portions of said main body, and connected electrically and mechanically to the respective end portions of said fusible member, and wherein said electrical and mechanical connection is executed by welding.
The third object of the present invention mentioned above may be accomplished by a miniature fuse of surface mount type according to the present invention including a fusible member, a main body made of heat resistant insulating material and a pair of conductive terminals, wherein said main body has a columnar configuration and a cavity defined inside of the main body between the opposite end portions, said fusible member is disposed in said cavity of said main body between said opposite end portions, the opposite end portions of said fusible member are extended outwardly onto the outer surface of said main body from the opposite end portions of said main body or from the vicinities thereof, the respective conductive terminals are fit onto the respective end portions of said main body and electrically connected to the respective end portions of said fusible member, and wherein said main body is comprised of two split members which are separated in the direction of connecting the opposite end portions; and recessed portions extending to the split end surface are provided, as the recessed portions of said main body, in the vicinities of the respective end portions of the side surfaces of at least one of said split members forming the columnar configuration of the main body, whereby workability in the course of manufacturing may be improved.
According to an aspect of the present invention, it is preferable that recessed portions extending to the split end surfaces are provided in the vicinities of the respective end portions of the side surfaces of the other of said split members forming the columnar configuration of the main body; and the recessed portions of two split members are adapted to form one recessed portion at the side surfaces forming the columnar configuration when said two split members are jointed to form said main body.
According to another aspect of the present invention, it is preferable that said conductive terminals are metallic caps; the end portions of said fusible members are connected to said caps by welding, and projections adapted to fit in the recessed portions of said main body are formed at said caps by said welding in order to fix said caps to said main body.
According to a further aspect of the present invention, said main body is preferably made of ceramic material.
According to the present invention, the support member is made of a material that has ease of shape formation, has pressure resistant strength, and contains in terms of weight ratio Al2O3 exceeding 96%, MgO exceeding 3%, and BeO less than 1%; the fusible member is made of a metallic material having a low melting temperature and containing in terms of weight ratio Ag equal to or exceeding 50%, Cu equal to or exceeding 20%, Zn equal to or exceeding 17% and Sn equal to or exceeding 5%; and the electrical and mechanical connection is executed by welding so that the pre-arcing time-current characteristic can be kept stable and a strong time lag characteristic can be provided and a high breaking capacity in the order of 60A at AC 600V without sustaining arcs due to metallic vaporization of the soldering materials can be attained.
Further according to the present invention, since the conductive terminals and the fusible member are connected by welding, a steady pre-arcing time-current characteristic and a large breaking capacity can be attained without uneven performance, and the miniature fuse of surface mount type may not be affected by the heat generated at the time of soldering the miniature fuse of surface mount type to a printed circuit board after assembly, whereby stable connection between them can be maintained at the time of mounting the fuse to the printed circuit board.
Still according to the present invention, since the main body is comprised of two split members which are separated in the direction of connecting the opposite end portions, and recessed portions extending to the split end surface are provided, as the recessed portions of said main body, in the vicinities of the respective end portions of the side surfaces of at least one of said split members forming the columnar configuration of the main body, such a construction as having a recessed portion at the side surface of the main body can be manufactured by press molding and the fusing member can be extended in a casing of a fuse easily, whereby production of miniature fuse of surface mount type can be made easy, automated production can also be facilitated and production rate can be enhanced.